Because of their exquisite specificity, biological molecules, including antibodies, have been employed in biosensors. Biosensors are devices capable of identifying and quantifying a target chemical. Biosensors are highly sensitive to their analyte (the chemical species to be detected for an antibody-based biosensor, the analyte is the antigen to the antibody). They are able to detect quantities as small as 10.sup.-15 gram. They are also extremely specific toward the analyte because of the unique ability of the antibodies to recognize their target species at the molecular level.
The present state of the art in antibody-based biosensors is illustrated by the various commercially available immunoassays. An immunoassay is a chemical test based on the use of antibodies to bind the molecule to be detected. In these assays, an antibody specific to the analyte (the "capture antibody") is immobilized onto a solid surface. This surface is then exposed to the sample to be analyzed and the immobilized antibodies bind some of the analyte present in the sample. After the surface is washed, it is immersed in a solution of a second antibody (the "signal antibody") specific to the same analyte. The signal antibody is conjugated (attached chemically) to a radioactive, fluorescent, or enzymatic label, so that it can be detected with high sensitivity. The amount of the signal antibody bound to the analyte is determined by the amount of radioactivity, intensity of fluorescence, or quantity of enzymatic reaction product, which in turn is proportional to the quantity of antigen in the sample. In the case of the enzyme label, the enzyme converts molecules of an added colorless reactant to colored reaction products. The intensity of the color change is read by a spectrophotometer. This type of assay is called enzyme-linked immunosorbent assay (ELISA). Examples of commercially available ELISA test kits are home pregnancy tests and environmental monitoring tests for BTEX (benzene, toluene, ethylbenzene, and xylene), PAH's (polynuclear aromatic hydrocarbons) or PCB's (polychlorinated biphenyls) in water. ELISA assays are also used in the military for battlefield detection of chemical and biological warfare agents. A disadvantage of these immunoassay kits is that a separate kit is required for each antigen or closely related family of antigens being tested for. Not only is this costly and labor consuming when many antigens must be tested for, but it can also result in dangerous time delays as when chemical and biological warfare agents are being tested for on the battlefield.
It would be desirable to provide a single device that could perform multiple immunoassay tests at the same time. The test results of such a device would be read and evaluated automatically. In order to achieve this, each type of antibody must be precisely and discretely located on the test surface. Cross contamination of the antibodies must be avoided. Moreover, such devices should be inexpensive and easy to manufacture.